Unusually mild summer fades into beginning of fall

Stockton enjoyed the coolest summer since 1999, with an average high temperature a hair below 90 degrees.

It was cooler than normal up and down the West Coast.

"The invisible summer, seamless from spring to fall," said Bill Patzert, a scientist at NASA's Jet Propulsion Laboratory who studies the role of oceans in the global climate.

Triple-digit days, too, were unusually rare in the Valley. San Joaquin County had eight of them, the fewest since 1997 (also eight days).

Multiple daily records were broken in Stockton, including, most recently, a record-low of 48 degrees early Sept. 15.

"The ocean never warmed," Patzert said. And the impact extended inland. The trend knocked the upper end off normal daytime summer highs in regions miles from the coast.

"It wasn't the coolest summer ever, that's for sure, but in a warming world, it was a little gift from the weather gods," Patzert said.

It might be a stretch, though, to call this a historically cool summer.

A number of years have been cooler: 1986, 1989, 1991, 1998 and 1999 to name a few.

But 2010 felt special - perhaps because we're still recovering from the 26 100-degree days last year.

August's statewide average was cooler than normal, and the month ended with a significant low-pressure system that even raised concerns that backcountry hikers in the Sierra Nevada might be caught unprepared by a summer snowfall.

The trend continued into September.

One benefit of the cool temperatures is less water use. Stockton East Water District has been treating 5 million gallons of water every day, thanks to a full supply from upstream rivers.

And yet, city dwellers have been using less water - perhaps in part because of the economy and vacant homes, and perhaps in part because lawns have required less water, and swimming pools aren't getting refilled as often thanks to lower evaporation rates.

All of this means that the city can decrease pumping from its wells, saving precious groundwater and instead can use the surface water from rivers and streams.

"We'll take it. We'll definitely take it," said Kevin Kauffman, general manager of Stockton East.

Mild temperatures could, however, be bad for farmers. Crop harvests are running late, which could be a problem if an early-season storm hits - just like in 2009, with the remnants of Typhoon Melor in mid-October.

"Everyone's holding their breath that we can get everything harvested," said Bruce Blodgett, head of the San Joaquin County Farm Bureau Federation. "Ultimately, we also need some rainfall, so we're hoping we'll get everything harvested, then let it rain, let it rain, let it rain some more."

What the coming months will bring is anyone's guess. La Niņa is strengthening in the Pacific Ocean, meaning cold ocean currents and a changing climate along the West Coast.

The Climate Prediction Center expects cooler-than-normal temperatures in the Pacific Northwest and much of Northern California this fall and winter.

And as for rainfall, La Niņa often means wet in the Pacific Northwest and dry in the Southeast.